A recent development in the field of metallurgy has been the production of metallic glasses. Metallic glasses comprise certain complex metal alloys which can be put into glass form, i.e., the bodies have a random atomic structure, by cooling melts of the alloys so rapidly that an organized crystal structure does not have time to develop. The production of such materials has involved forms of rapid melt quenching or various condensation processes, e.g., splat cooling, vapor deposition, electrodeposition, and sputtering. This requirement of rapid cooling has resulted in the newly-formed glasses being very small in at least one dimension, i.e., the bodies have commonly been in the shape of ribbons, flakes, wires, films, or powders. Thus, the largest articles formed from metallic glasses of particular alloy compositions have been thin sheets having a thickness of about 0.01-0.05 inches and about 25-65 mm in width.
Metallic glasses demonstrate magnetic and mechanical properties of great commercial potential. Iron-containing alloys have received much attention because of their exceptional ferromagnetic properties. With regard to mechanical properties, ribbons of certain metallic glasses have displayed extremely high fracture strength, i.e., approaching their theoretical strength, with highly localized shear deformation being observed to precede the tensile fracture. This phenomenon is in marked contrast to the brittle fracture behavior manifested by non-metallic glasses. In the latter, the fracture is characterized by crack initiation and propagation.
The density of normal liquid metals is about 5% less than that of the crystalline phase at the melting temperature. Based upon the difference in thermal expansion between liquid and crystalline metals, the density of metallic glasses at their transition temperatures would approach within 2% of the crystalline value and this circumstance has, indeed, been observed. Contrariwise, most non-metallic glasses and bodies formed through random, hard sphere packing exhibit densities that are about 15% less than those of the close-packed structure. This phenomenon can be attributed to the character of the metallic bonding which is such that the energy of a system is dominated by the average atomic volume, rather than the atomic distance.
The random atomic structure of metallic glasses is responsible for imparting unusual properties to them. For example, the materials are typically much stronger than crystalline metals, shear moduli in excess of 50 being reported on some compositions. Their essential insensitivity to many types of radiations, such as that from neutrons, has been noted. Moreover, in many instances, the metallic glass has been reported as demonstrating much greater corrosion resistance than the corresponding cyrstalline alloy.
However, practical application of metallic glasses has been severely limited because of the above-observed obstacle of body size in which the glasses have been produced. Hence, because these materials exhibit both a high diffusivity at the melting temperature and a relatively low glass transition temperature, the metal liquids customarily crystallize when cooled at rates at which some non-metallic liquids form glasses. Consequently, non-crystallized metals can only be prepared via drastic quenching techniques. Those factors giving rise to the crystallization of metals during conventional cooling of melts have also prevented the formation of bulk bodies of metallic glasses from the original powders, ribbons, films, etc., utilizing conventional forming techniques. Thus, when metallic glasses are heated to a point about half of their melting temperatures, they begin to lose their random structure, i.e., they begin to crystallize, and thereby lose their unique properties.
One solution which has been proposed to solve that problem has been to fuse or weld the finely-dimensioned starting materials together so quickly that crystallization does not have time to occur. The use of chemical explosives to force the materials together so quickly that heat buildup does not occcur has been tried with some success. Thus, simple shapes such as rods, plates, tubes, and cones have been prepared in this manner. Nevertheless, it is apparent that cost and technique complexity severely limit the application of that practice.